Because they are reaching outside of their state to regulate. Pretty easy to understand.Why do you think you are exempt from that state's rules? Why doesn't that state get to regulate operations within their borders?
They aren't taxing anyone outside of their state. They are making them collect taxes for them.They aren't just arbitrarily taxing people in another state. You are operating in their borders by choice. That choice comes with certain obligations. Feel free to not sell to people in that state.